And, the muscles don't return to normal even after the astronaut is back on Earth for several weeks, researchers from the University of California, San Diego, discovered.

But there were no changes in astronauts' spinal disc height, according to the study.

The researchers assessed six NASA astronauts who spent four to seven months on the International Space Station. The researchers said the study offers new insight into increased rates of back pain and spinal disc disease among astronauts on long space missions.

"These findings run counter to the current scientific thinking about the effects of microgravity on disc swelling," said study author Dr. Douglas Chang. He's an associate professor of orthopedic surgery and chief of physical medicine and rehabilitation service at UC San Diego Health.

"Further studies will be needed to clarify the effects on disc height, and determine whether they contribute to the increase in body height during space missions, and to the increased risk of herniated discs," Chang said in a university news release.

"However, it's information like this that could provide helpful information needed to support longer space missions, such as a manned mission to Mars," he added.

He said the results suggest possible ways to reduce the effects of spaceflight on the spine. For example, core-strengthening exercises that are recommended for patients with back pain might benefit astronauts in training.